The present disclosure relates generally to electronic communications systems and, more particularly, to broadband linearization by elimination of harmonics and intermodulation in amplifiers.
In electronic communication systems, it is often the case that groups of information signals are amplified and transmitted simultaneously. For example, a cellular radio base station transmitter may transmit signals to several active receiving mobile stations within a single geographic cell. These signals may appear at multiple predetermined frequencies in such multi-carrier signals. Similarly, a satellite communications transponder may amplify and transmit a large number of information signals destined for various participating remote stations. Some communications systems may employ an encoding scheme such as frequency division multiple access (FDMA), in which information signals are modulated on signal carriers occupying several frequency channels within an allocated frequency band. Here, measures are taken so as to avoid inter-channel interference which may corrupt signal transmissions.
One possible source of such cross-channel interference is known as intermodulation distortion (IMD), which may result when two or more signals of different frequencies are mixed. For example, if two carriers of different frequencies are amplified using a non-linear amplifier, spurious outputs may occur at the sum and difference of integer multiples of the original carrier frequencies. IMD tends to cause problems in transmitters that amplify and send out multicarrier signals, as it often falls in the spectrum between two of the carrier signals. Further, IMD is generally difficult to filter out, because such a filter would likely also filter out the carrier signal, removing that signal and the associated data from the signal that is ultimately amplified by the power amplifier. In addition to environmental and aging factors, the observed IMD performance of a power amplifier is sensitive to changes in output power, the number of carriers and their frequency separation.
Existing approaches to suppressing intermodulation and distortion products have come at a cost of lower power efficiency, higher complexity and component cost, and/or effectiveness for narrow band systems only.